Variable-gain gain-clamped optical amplifiers

ABSTRACT

An optical amplifier is disclosed comprising a signal semiconductor optical amplifier having a waveguide, forming at least part of a signal path between an input and an output, extending along a signal active region for amplification of a signal. The amplifier also includes a control active region of semiconductor material having a gain which is controllable independently from the gain of the signal active region. The amplifier also includes a laser cavity containing both the signal active region and the control active region and being capable of clamping the gain of the signal active region, and the control active region is arranged not to amplify a signal in the signal path within a predetermined signal band.

PRIORITY CLAIMS

This application claims priority to GB 0121466.7 filed on Sep. 5, 2001,GB 0123553.0 filed on Oct. 1, 2001, and the United States was timelyspecified as an Elected Country in the PCT application no.PCT/GB02/04061, filed Sep. 5, 2002, under 35 U.S.C. 119 which isexplicitly incorporated by reference as if set forth below.

BACKGROUND

The present invention relates to optical amplifiers using semiconductoroptical amplifiers (SOAs) to amplify an optical signal. Such SOAs havewide applications in telecommunication networks as low-cost linearamplifiers for optical data signals. In particular the present inventionrelates to the use of a laser cavity to provide gain-clamping forincreasing the linear region of operation and hence reducinginterference noise.

Operation of a SOA outside the linear region of operation causesnon-linear distortion and interference noise. In particular, at highoutput powers the gain reduces. Such gain modulation can causenon-linear distortion and interference noise in the time domain, that isinter-symbol interference, because the gain recovery time of a SOA istypically similar to the data modulation speed. Similarly, such gainmodulation can cause interference noise in the frequency domain, that isinter-channel crosstalk between different frequency channels.

Gain-clamping using a laser cavity is a known technique to reduce gainmodulation and the associated interference noise. Typically a lasercavity containing the active material of the SOA is provided to lase ata wavelength outside the desired signal band. In one known arrangement,wavelength dependant reflectors are arranged before and after the SOA toform a laser cavity longitudinally along the signal path. In anotherarrangement disclosed in U.S. Pat. No. 5,436,759, distributed Braggreflectors are arranged to form a laser cavity extending vertically,that is perpendicular to the signal propagation axis and the layeredstructure of the SOA.

The lasing action in the laser cavity clamps the gain of the activematerial at the laser threshold. A clamped gain is therefore imposed onthe amplification of signals in the signal band. By clamping the gain ata level below the normal unclamped gain, the linear region is extendedto higher output powers. In particular, the linear region is extended tothe saturation output power which, for a given bias current, has ahigher level at the lower, clamped gain than at the higher, unclampedgain. This reduces the gain modulation and interference noise.

Gain clamping occurs at the expense of gain variability in that the gainis clamped at a value fixed at the point of manufacture. However itwould be desirable provide a gain-clamped SOA for which the level of theclamped gain is controllable in use. There are many situations intelecommunications where this is desirable, for example to allow controlthe noise figure for the SOA which increases as the gain and hence thecarrier density is reduced. Control of the clamped gain is alsoimportant in applications such as channel equalization and automaticpower control. Automatic power control is becoming increasinglyimportant within optical receivers to optimize the signal strength atthe detector, particularly at data rates in excess of 10 GHz whereautomatic power control in the electronic domain can be prohibitivelyexpensive.

The gain of a SOA which is not gain-clamped can be controlled bychanging the bias current supplied to the SOA. However if the biascurrent is lowered to lower the gain, the saturation output power andhence the linear region also reduces. This contrasts with clamping thegain to a lower level using a laser cavity, in which case the saturationoutput power is increased at a given bias current, as explained above.What would be desirable is to provide for gain-clamping with variabilityof the gain at which clamping occurs so as to maximize the saturationoutput power.

According to the present invention, there is provided an opticalamplifier comprising: a signal semiconductor optical amplifier having awaveguide forming at least part of a signal path between an input and anoutput, extending along a signal active region for amplification of asignal; a control active region of semiconductor material having a gainwhich is controllable independently from the gain of the signal activeregion; and a laser cavity containing both the signal active region andthe control active region and being capable of clamping the gain of thesignal active region, wherein the control active region is arranged notto amplify a signal in the signal path within a predetermined signalband.

In use, the signal active region amplifies the signal in a predeterminedsignal band passing along the signal path. The laser cavity fixes thetotal gain of both active regions because they are both contained in thelaser cavity. The total gain is clamped at the laser threshold for thelaser cavity, that is where the total gain is equal to the losses of thelaser cavity, this being inherent in the lasing action.

In use, this arrangement allows the gain of the signal active region tobe varied by controlling the gain of the control active region, forexample by controlling the bias current supplied to the control activeregion. As the total gain within the lasing mode is limited to the losswithin the cavity, changing the gain of the control active region causesan opposite change in the gain of the signal active region at thewavelength of the lasing mode. Thus a changed clamped gain is alsoimposed on the amplification of signals in the predetermined signal bandwhich pass in the signal path through the signal active region.

Furthermore, as the control active region does not amplify a signal inthe signal path within a predetermined signal band, it is possible tochange the bias current to the control active region, or otherwisechange the gain of the control active region, without changing the biascurrent supplied to the signal active region. Thus, the opticalamplifier in accordance with the present invention allows signals to beamplified by the signal active region at a clamped gain which isvariable by control of the control active region, so maximizing thesaturation output power and hence the linear range.

Although the saturation output power of the control active region ischanged, this has no effect on the saturation output power of the signalactive region.

It is not necessary to change the bias current to the signal activeregion, although it is in principle possible to do so. Preferably, thesignal active region is supplied with a maximum bias current in order tomaximize the saturated output power for the signal channel.

Preferably, the laser cavity has a lasing mode at a wavelength outsidethe predetermined signal band. This is desirable to reduce the effect ofthe lasing mode on the signal passing through the signal active region,given that the lasing mode also propagates through the signal activeregion.

To control the wavelength of the lasing mode, the laser cavity mayinclude a wavelength-dependent element, for example, a filter in thelasing cavity outside the signal path or a wavelength-selective couplerwithin the laser cavity, or wavelength-dependent reflectors to terminatethe laser cavity.

The present may be embodied by several different types of opticalamplifier.

In a first type of embodiment the control active region is the activeregion of a control SOA formed in a separate semiconductor chip from thesignal SOA. This type of embodiment allows the optical amplifier to beconstructed from optical components which, in themselves, are of knownconstruction.

In a second type of embodiment, the signal active region and the controlactive region are different portions of the same semiconductor chip withthe waveguide extending along both the signal active region and thecontrol active region. This may be achieved, for example by at least oneof the electrodes of the signal and control active regions beingelectrically isolated between the signal and the control active regions.One benefit of this arrangement is that it allows the entire opticalamplifier to be integrated in a single semiconductor chip. Apart fromthe electrode configuration, the signal and control active region may inthemselves have a conventional construction for an SOA. This allows thepresent invention to be easily applied to known SOA constructions. Itresults in an SOA having fundamentally the same properties as a knownconstruction of SOA, but with a variable clamped gain.

In a third type of embodiment, the control active region is integratedoutside the waveguide in the same semiconductor chip as the signalsemiconductor optical amplifier. In a simple arrangement, the lasercavity extends transversely to the waveguide, preferably perpendicularto the layered structure of the semiconductor chip. This allows thesignal and control active regions to be formed by respective layers ofactive material. In itself the laser cavity may have the construction ofa known VCSEL (Vertical Cavity Surface Emitting Laser). This allows theprovision of a relatively short laser cavity which provides severaladvantages including a quick response time. This type of embodiment alsoprovides the advantage that the entire variable-gain gain-clampedoptical amplifier may be integrated in a single semiconductor chip.Furthermore, the provision of the active regions as separate layersprovide a high degree over of control over the characteristics andproperties of the two active regions.

Two different techniques are used to prevent the control active regionfrom amplifying a signal passing through the signal active region.

The first technique is for the control active region to be outside thesignal path so that is does not amplify a signal in the signal path.This is intrinsic in the third type of embodiment. It may be implementedin the first type of embodiment by forming part of the laser cavity anoptical path which is outside the signal path and in which the controlactive region is arranged. To couple that optical path into the signalpath, in particular to the portion of the signal path containing thesignal active region, one or more optical couplers may be provided inthe signal path. The optical coupler(s) may be of any suitable form.

One preferred form is to use a wavelength-selective coupler such as awavelength division multiplexing (WDM) coupler. This is preferred as thelowest loss implementation because the power of the signal path is notlost by being coupled into the laser cavity. The use of such awavelength-selective coupler on the output side of the signal SOA alsohas the advantage that it prevents any transmission of the lasing modeonto the signal path towards the output of the optical amplifier.

As an alternative, the optical coupler(s) may be awavelength-insensitive coupler such as a weighted beam splitter whichsplits (or combines) light at all wavelengths in a predetermined ratio.Such an arrangement is lossy, because some of the lasing mode is coupledinto the signal path outside the laser cavity and some of the signal iscoupled into the laser cavity, but may nonetheless be acceptable ifthere is sufficient gain in the lasing path. There is a cost-benefit inthat wavelength-insensitive couplers are less expensive.

Various configurations for the laser cavity may be used. One simpleconfiguration is a ring laser cavity in which the optical circuitprovides a ring-shaped cavity around which the laser mode propagates. Inthis case, the laser cavity may include an isolator, or otherdirectional element, to control the propagation direction of the lasingmode to be co-directional or counter-directional with respect to thesignal path. Alternatively, the laser cavity may contain no directionalelements to allow the lasing mode to propagate in both directions aroundthe ring laser cavity. Control of the directionality of the laser cavitywith respect to the signal path allows the optical power density to beequalized along the signal path and also gives control over the amountof amplified spontaneous emission (ASE) that is transmitted in theforward direction on the signal path.

To couple the ring laser into the signal path, the optical circuit mayinclude a pair of optical couplers in the signal path on the input sideand output side, respectively, of the signal semiconductor opticalamplifier. In this case, the pair of optical couplers may couplerespective ends of an optical path which contains the controlsemiconductor optical amplifier and which, together with the portion ofthe signal path between the pair of optical couplers, forms the ringlaser cavity.

Another configuration for the laser cavity is a linear laser cavity inwhich the lasing mode is reflected at either end of the cavity topropagate back and forth. To couple the linear laser cavity into thesignal path the optical circuit may include an optical coupler in thesignal path. In this case, the optical coupler may couple an end of anoptical path which contains the control semiconductor optical amplifierand which is terminated by a reflector to form one end of the linearlaser cavity. The optical path containing the control SOA thereforeforms part of the laser cavity, together with a portion of the signalpath extending from the optical coupler and containing the signal SOA.

The other end of the linear laser cavity is preferably terminated by areflector, such as a Bragg reflector, arranged in the signal path,although it is possible to terminate the laser cavity by a reflectorarranged outside the signal path and coupled into the signal path by asecond optical coupler.

The second technique to prevent the control active region fromamplifying a signal passing along the signal path the controlsemiconductor optical amplifier has an insignificant gain in thepredetermined signal band so that it does not amplify a signal in thepredetermined signal band. This allows the control semiconductor opticalamplifier to be arranged in the signal path which is beneficial becauseit allows for the optical amplifier to have a simpler construction. Forexample, the laser cavity may be terminated by reflectors arranged inthe signal path outside the control and signal SOAs. The secondtechnique is intrinsic in the second type of embodiment. However thesecond technique may be applied in combination with a first technique toparticular advantage.

With the second technique, the control active region has a significantgain at the wavelength of the lasing mode, but an insignificant gain inthe predetermined signal band. This may be achieved by selection of thegain profile of the control active region with respect to the gainprofile of the signal active region which will be significant, andusually peak near the predetermined signal band. In particular, this maybe achieved by providing the gain profile of the control active regionto overlap with the gain profile of the signal active region at thewavelength of the lasing mode.

To allow better understanding, embodiments of the present invention willnow be described by way of non-limitative example. With reference to theaccompanying drawings, in which:

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an SOA, the cross-section beingtaken across the SOA perpendicular to the optical axis;

FIG. 2 is a graph of the gain profile of a signal SOA;

FIG. 3 is a diagram of an optical amplifier employing a ring lasercavity;

FIG. 4 is a graph illustrating the multiplexing bands of a WDM coupler;

FIG. 5 is a graph of the gain characteristic for gain-clamped opticalamplifiers in accordance with the present invention;

FIGS. 6 to 8 are diagrams of further optical amplifiers employing a ringlaser cavity;

FIGS. 7 to 13 are diagrams of optical amplifiers employing a linearcavity;

FIG. 14 is a diagram of an optical amplifier in which the control SOA isin the optical path;

FIG. 15 is a graph of the gain profile of the signal and control SOAs inthe optical amplifier of FIG. 14;

FIG. 16 is a perspective view of an optical amplifier integrated in asingle semiconductor chip;

FIG. 17 is a sectional view of a portion of a first optical amplifierhaving a laser cavity transverse to the signal path; and

FIG. 18 is a sectional view of a portion of a second optical amplifierhaving a laser cavity transverse to the signal path;

The various embodiments described below share a number of commonelements which perform the same function in each embodiment. To avoidrepetition, such common elements are referred to using the samereference numerals and a description thereof is not repeated.

DETAILED DESCRIPTION

Firstly, embodiments of a first type will be described in which theoptical amplifier of the invention comprises a signal SOA 1 and acontrol SOA 2 arranged in an optical circuit.

The signal and control SOAs 1, 2 may be any known form of SOA having awaveguide which passes light through active material which amplifies thelight. In this type of embodiment, the active semiconductor material ofthe signal and control SOAs 1, 2 arranged along the waveguides of thesignal and control SOAs 1, 2 constitutes the signal and control activeregions respectively. SOAs 1, 2 are referred to by the labels “signal”and “control” to distinguish between them, but may be the same type ofdevice. A suitable semiconductor optical amplifier is described inWO-96/41405.

Another suitable structure for the control and signal SOA 1, 2 suitablefor use within telecommunications networks, will now be described withreference to FIG. 1. FIG. 1 is a cross-sectional view across the activeregion of the SOA 1, 2, perpendicular to the optical axis.

The SOA 1, 2 is formed as a single semiconductor wafer having acrystalline structure of semiconductor materials. Further SOAs may beformed in the same wafer. The base materials are In and P to provide again characteristic centered nearer the 1550 nm band of channels, butalternatively other semiconductor materials could be used. The SOA 1, 2has a layered construction of successive layers and may be manufacturedby a deposition process such as MOCVD (metalorganic chemical vapordeposition) or MBE (molecular beam epitaxy) together with lithographictechniques, such manufacture being in itself conventual. The growth maybe performed to provide each layer with the necessary physicalcharacteristics and also to allow accurate control of the thickness ofeach layer. The layers will now be described in more detail.

A substrate 101 of base material InP is provided with a greaterthickness than the other layers to impose a crystal lattice on thoseother layers.

Above the substrate 101, the SOA 1, 2 has a waveguide structurecomprising a active layer 102 constituted by active semiconductormaterial, that is InGaPAs in appropriate proportions. The active layer102 have a width less than that of the substrate 101 and is surroundedby various layers of material, typically InP, having a lower refractiveindex than the active layer 102 to create optical confinement of lightwithin the active layer 102 which therefore act as a waveguide. Theactive layer 102 has suitable dimensions to support a single mode in itsoperating frequency band.

Between the substrate 101 and the active layer 102, the SOA 1, 2 has abuffer layer 103 of n-type InP adjacent the substrate 101 and a lowercladding layer 104 of InP. The lower cladding layer 104 is stepped toprovide a protruding portion 104 a of substantially the same width asthe active layer 102 and on which the active layer 102 is formed.

Above the active layer 102, the SOA 1, 2 has an upper cladding layer 105of InP adjacent the active waveguide 102 and a cap layer 106 of p-typeInP. Both the upper cladding layer 105 and the cap layer 106 are ofsubstantially the same width as the active layer 102.

Although the active layer 102 is of substantially the same width as theadjacent lower and upper cladding layers 104, 105, and the active layer102 is preferably notch-etched during manufacture which is aconventional technique to remove surface defects. Such notch-etchingslightly reduces the width of the active layer 102 so that the adjacentlayers overhang.

The SOA 1, 2 further has a first current blocking layer 107 of n-typeInP provided above the lower cladding layer 104 and covering the sidesof the active waveguide 102 and the upper cladding layer 105 andextending at least partially over the sides of the cap layer 106,preferably to the upper surface of the cap layer 106. A second currentblocking layer 108 of p-type InP covers the first current blocking layer107.

The second current blocking layer 108 and the cap layer 106 are coveredby an overgrowth layer 109 of InP. On the upper surface of theovergrowth layer 109, a contact layer 110 of InGaAs is provided.

The SOA 1, 2 has an upper contact 111 in electrical contact with thecontact layer 110 and a lower contact 112 in electrical contact with thesubstrate 101. The upper and lower contacts 111 and 112 are made ofmetal, such as gold.

For electrical isolation, an insulator layer 113 is provided extendingacross the upper surface of the overgrowth layer 113. The insulatorlayer 113 extends under the upper contact 111 overlapping the edges ofthe contact layer 110. Thus the upper contact 111 contacts the contactlayer 110 in a window between the edges of the insulator layer 113.

In use, a drive current is passed between the upper contact 111 and thelower contact 112. The drive current pumps the material of the activelayer 102 which therefore amplifies the light passing along the activewaveguide formed by the active layer 102. The SOA 1, 2 may have thestructure illustrated in FIG. 1 along its entire length, although othermore complicated structures are possible. The end facets of the SOA 1, 2are arranged to have a very low reflection, typically of the order of10⁻³ or 10⁻⁴, for example by being formed with anti-reflection coatings.

The described embodiments all have a single signal SOA 1 and a singlecontrol SOA 2, but in principle nay number of signal SOAs and any numberof control SOAs could be provided in series or parallel with each other.

In use, the active material of the signal SOA 1 amplifies signals in adesired signal band carrying data within a telecommunications network.FIG. 2 illustrates a typical gain profile for the signal SOA 1 and is agraph of relative optical density (dB) against wavelength (nm). Thussignal SOA 1 has a gain profile which peaks within the desired signalband 3.

The optical paths of the respective optical circuits are formed bypassive waveguiding structures formed in separate elements from thesignal SOA 1 and the control SOA 2, such as silica-on-silicon, siliconor polymer waveguides which are readily commercially available. Thisallows all the elements of the optical amplifier, including the signalSOA 1 and the control SOA 2, the waveguides and the other elements ofthe optical amplifier, to be mounted in an integral form on a commonmicrobench to provide the optical amplifier as a single module. However,this is not essential and the optical paths may be formed by waveguidesof any suitable form, such as optical fibers coupled between the variouselements.

Thus, the embodiments of the first type may be constructed from opticalelements including the signal and control SOAs 1, 2, which are inthemselves of known construction. This allows the present invention tobe implemented in a convenient manner using readily availablecomponents. That being said, the optical circuits could be integratedinto a single semiconductor chip in which case the optical paths arewaveguides formed within the semiconductor by conventional manufacturingtechniques such as epitaxy.

The control SOA 2 does not amplify the signal in the signal band 3. Thismay be achieved using two alternative techniques. The first technique isfor the control SOA 2 to be outside the signal path. The secondtechnique is for the control SOA 2 to have an insignificant gain in thepredetermined signal band in which case the control SOA may be in thesignal path. Embodiments using the first technique will now bedescribed. In each of the following embodiments, the second technique ofproviding the control SOA 2 to have an insignificant gain in thepredetermined signal band may be used to particular advantage incombination with the first technique of providing a control SOA 2outside the signal path.

FIG. 3 illustrates an optical amplifier in which the signal SOA 1 andthe control SOA 2 are arranged in an optical circuit including a ringlaser cavity containing both the signal SOA 1 and the control SOA 2. Thesignal SOA 1 is arranged in a signal path 4 between an input 5 and anoutput 6. Optical couplers 7 and 8 are arranged in the signal path 4 onopposite sides of the signal SOA 1. The optical couplers 7 and 8 couplean optical path 9 containing the control SOA 2 into the signal path 4 toform a ring cavity consisting of both the optical path 9 and the portion10 of the signal path 4 between the optical couplers 7 and 8. Thus theoptical couplers 7 and 8 couple between, on one hand, the portion 10 ofthe signal path 4 between the optical couplers 7 and 8 and, on the otherhand, both the remainder of the signal path 4 outside the ring cavityand optical path 9.

The ring cavity is arranged to lase at a wavelength outside the signalband 3. For example, FIG. 2 illustrates a typical lasing mode 11 on thegain profile of the signal SOA 1 adjacent the signal band 3. Byproviding the lasing mode 11 outside the signal band 3, the lasing mode11 may be removed from the signal transmitted to the output 6, forexample by filtering or by the optical coupler 8 on the output side ofthe signal SOA 1 being wavelength-selective, as further described below.

In general, the wavelength of the lasing mode 11 may be controlled byany wavelength-dependent element in the laser cavity. However, in theoptical amplifier of FIG. 2, the wavelength-dependent elements are theoptical couplers 7 and 8 which are wave division multiplexing (WDM)couplers or other wavelength-selective couplers. In particular, theoptical couplers 7 and 8 multiplex together (i) light at the wavelengthof the lasing mode on optical path 9 and (ii) light in the signal band 3on the remainder of the signal path 4 outside the ring cavity. Themultiplexed light is coupled to the portion 10 of the signal path 4between the optical couplers 7 and 8. For example, FIG. 4 illustratesthe power spectrum for the channels of the optical couplers 7 and 8,being a graph of relative power intensity (db) of the light on theportion 10 of the signal path 4 within the ring cavity againstwavelength (nm). The pass band 12 of the optical couplers 7 and 8 isarranged to contain the signal band 3 including data 13 and is coupledto the remainder of the signal path 4 outside the ring cavity. Thereflect band 14 is arranged to contain the lasing mode 11 and is coupledto the remainder of the optical path 4 outside the ring cavity.

An isolator 15 is provided in the optical path 9 outside the signal path4. The isolator 15 controls the propagation direction of the lasing modein the ring cavity to be counter-directional with respect to the forwarddirection along the signal path 4 between the input 5 and the output 6.This is beneficial because it reduces the amount of ASE transmitted inthe forward direction along the signal path 4.

Data in the signal band 3 passes along the signal path through thesignal SOA 1 only once due to the action of the optical couplers 7 and8. Wavelengths that are outside the signal band 3 are passed by the WDMcouplers 7 and 8 around the ring cavity many times, the number of passesbeing dictated by net gain of the ring cavity. This light forms thelasing mode. The ring cavity can lase at any wavelength passed by theoptical couplers 7 and 8, but in practice the wavelength of the lasingmode will be where the gain is greatest (or the loss is lowest), forexample at the wavelength where the reflect band of the optical couplers7 and 8 intersects with the overall gain profile of the SOAs 1 and 2.

In use, the lasing action clamps the total gain of the signal SOA 1 andthe control SOA 2 at that gain where the lasing threshold is reached, inother words where the total gain equals the total losses of the lasercavity. Thus the gain of the signal SOA 1 in the signal band is alsoclamped.

In use, the gain of the control SOA 2 is controlled. This may be done bycontrolling the drive current supplied to the control SOA 2.

Alternatively the gain of the control SOA 2 may be controlled thermally.To achieve this the control SOA 2 is provided with resistive paths whichheat the active material of the control SOA 2 when a current is passed.The substrate of the control SOA 2 effectively acts as a heat sink at aconstant temperature. Therefore the temperature of the active materialof the control SOA 2 is controlled by passing a current through theresistive paths and varying the level of the current.

As a consequence of varying the gain of the control SOA 2, the lasingaction of the laser cavity causes the clamped gain of the signal SOA 1at the lasing wavelength to vary oppositely to the gain of the controlSOA 2 so that the total gain of both SOAs 1 and 2 remains constant.Thus, the clamped gain of the signal SOA 1 imposed on amplification ofsignals in the signal band passing along the signal path 4 is alsovaried. In other words, control of the control SOA 2 allows the clampedgain of the signal SOA 1 in the signal band to be varied withoutvariation of the bias current supplied to the signal SOA 1. Thismaximizes the saturation output power of the signal SOA 1, as comparedto a corresponding change in gain achieved by varying the bias current.Although changing the bias current of the control SOA 2 affects thesaturation output of the control SOA 2, this does not limit thesaturation output power of the signal SOA 1. In normal use, the biascurrent supplied to the signal SOA 1 is fixed, usually at its maximumvalue in order to maximize the saturation output power.

In fact, the gain of the signal SOA 1 may even be varied into loss bycontrolling the bias current of the control SOA 2 to raise the gain ofthe control SOA 2 above the laser threshold of the laser cavity. This isadvantageous because there are many circumstances in atelecommunications network when it is desirable to reduce the level of adata signal, for example to bring it within the operating conditions ofa particular device. However such a linear reduction in power is notpossible with a simple SOA.

As an example, FIG. 5 shows the gain characteristic and is a graph ofthe gain (dB) of the signal SOA 1 against output power (dBm). The gainof the control SOA 2 if it were not gain-clamped is illustrated by theline 16 and has a linear region 17 extending up to the saturation outputpower above which the gain drops. The clamped gain of the signal SOA 1for different bias currents of the control SOA 2 is shown by the lines18. The clamped gain 18 is lower than the linear region 17 of theun-clamped gain 16. The clamped gain 18 for any given bias current ofthe control SOA 2 is linear up to the saturation output power of thesignal SOA 1 which occurs at a higher value than the saturation outputpower for the un-clamped gain 16.

FIG. 6 illustrates a further optical amplifier which is identical to theoptical amplifier of FIG. 3 except that the isolator 15 is arranged inthe opposite direction within the optical path 9 to control thepropagation direction of the lasing mode to be co-directional with theforward direction of the signal path 4. Similarly, FIG. 7 illustrates afurther optical amplifier which is identical to the optical amplifier ofFIG. 3 except that the isolator 15 is omitted. As a result, the lasingmode of the ring cavity propagates bi-directionally along the signalpath 4. Thus, the provision of the isolator 15 allows the propagationdirection of the lasing mode to be controlled. This in turn allows theoptical power density to be equalized along the signal path 4 of thesignal SOA and also gives control over the amount of ASE that istransmitted in the forward direction along the signal path 4.

FIGS. 8 to 10 illustrate further optical amplifiers which are identicalto the optical amplifiers of FIGS. 3, 6 and 7, respectively, except thata filter 19 is additionally provided in the optical path 9. The filter19 acts as wavelength-dependent element to control the wavelength of thelasing mode. The filter 19 also has the advantage of limiting the amountof ASE that is coupled back into the signal SOA 1. Such ASE injectedinto the signal SOA 1 would otherwise compromise the signal-to-noiseratio within the signal path 1.

In the embodiments described above, the optical couplers 7 and 8 are WDMcouplers. Such wavelength-selective couplers are preferable from thepoint of view of minimizing losses, since they prevent the lasing modebeing coupled out of the laser cavity and similarly they prevent thesignal being coupled out of the signal path 4. However, as analternative, the optical couplers 7 and 8 may each be replaced by awavelength-insensitive coupler, for example an appropriately weightedbeam splitter which couples and decouples beams at all wavelengths in apredetermined ratio. Such a beam splitter is acceptable provided thereis sufficient gain in the laser cavity to reach threshold even with theadditional losses. Typically, the beam splitter will be arranged tocouple 75% or more, typically 90%, of the incident light onto the signalpath 4 outside the ring cavity to maintain high efficiency in theforward direction along the signal path 4. A wavelength-insensitivecoupler provides a cost benefit, because such couplers are lessexpensive than wavelength-selective couplers such as WDM couplers.

FIG. 11 illustrates an optical amplifier in which a signal SOA 1 and thecontrol SOA 2 are arranged in an optical circuit including a linearlaser cavity containing the signal SOA 1 and the control SOA 2. Thesignal SOA 1 is arranged in a signal path 4 between an input 5 and anoutput 6. An optical coupler is arranged in the signal path 4 to couplean optical path 21 containing the control SOA 2 into the signal path 4.In the optical amplifier of FIG. 11, the optical coupler 20 is arrangedon the output side of the signal SOA 1, but this is not essential. Theoptical coupler 20 takes the same form as the optical couplers 7 and 8of the optical amplifiers employing a ring cavity described above withreference to FIGS. 2 and 6 to 10.

The optical path 21 is terminated by a reflector 22 so that the opticalpath 21 forms one end of a linear laser cavity. The reflector 22 maytake any suitable form, for example a simple reflective layer, a loopmirror or a Bragg grating. Advantageously, the reflector 22 may beintegrated into the semiconductor structure of the control SOA 2.

The opposite end of the laser cavity is formed by a reflector 23arranged in the signal path 4 on the input side of the signal SOA 1.Therefore, a linear laser cavity is formed between the reflectors 22 and23 by the optical path 21 together with the portion 24 of the signalpath 4 between the optical coupler 20 and the reflector 23. Thereflector 23 may be integrated within the semiconductor structure of thesignal SOA 1.

The reflector 23 is preferably a wavelength-dependent reflector such asa Bragg grating. In this case reflections of the signal in the signalband 3 passing along the signal path 4 are minimized, and also thereflector 23 acts as a wavelength-dependent element to control thewavelength of the lasing mode 11. Thus it is not essential for thereflector 22 also to be wavelength-dependent, although this may bedesirable. In addition, a filter 19 may optionally be provided in thesignal path 21 as a wavelength-dependent element to control thewavelength of the laser mode in the same manner as in the opticalamplifiers of FIGS. 8 to 10 employing a ring cavity.

The operation of the optical amplifier of FIG. 11 is the same as theoperation of the optical amplifier of FIG. 3 except that the lasercavity is linear rather than being a ring.

Many other forms of linear cavity are envisaged. For example, FIG. 12illustrates an optical amplifier which is identical to the opticalamplifier of FIG. 11 except that to terminate the laser cavity, insteadof the reflector 23 provided in the signal path 4, an optical coupler 25is provided in the signal path 4 on the input side of the signal SOA 1coupled to a further optical path 26 terminated by a further reflector27.

FIG. 13 illustrates a further optical amplifier having an identicalstructure of the optical amplifier of FIG. 11 except that the componentsforming the laser cavity are arranged on opposite sides of the signalSOA 1. Therefore, the optical coupler 20 is arranged on the input sideof the signal SOA 1 but coupled to an optical path 21 containing thesame elements including the control SOA 2. Similarly, the reflector 23is arranged in the signal path 4 on the output side of the signal SOA 2.Again, the reflector 23 could be replaced by an optical coupler 25,further optical path 26 and further reflector 27 as in the opticalamplifier of FIG. 12.

As an example of an embodiment employing the second technique ofproviding the control SOA 2 with an insignificant gain in the signalband, FIG. 14 illustrates an optical amplifier in which both the signalSOA 1 and the control SOA 2 are arranged in the signal path 4. Thecontrol SOA 2 is arranged to have an insignificant gain in the desiredsignal band 3 to prevent the control SOA 2 amplifying signals passingalong the signal path 4. However, the control SOA 2 has a significantgain at the wavelength of the lasing mode 11.

The optical amplifier of FIG. 14 further includes reflectors 28 arrangedin the signal path 4 on opposite sides of the two SOAs 1 and 2 to formtherebetween a linear laser cavity containing the signal SOA 1 and thecontrol SOA 2. The reflectors 28 are preferably wavelength-dependentreflectors such as Bragg gratings in order to act aswavelength-dependent elements to control the wavelength of the lasingmode 11 and also to prevent reflection of the signal passing along thesignal path 4.

As an example, the gain profiles of the signal SOA 1 and control SOA 2are illustrated in FIG. 15 which is a graph of relative opticalintensity (dB) against wavelength (nm). The gain profile of the signalSOA 1 is illustrated by the line 29 and has the same form as illustratedin FIG. 2 peaking around the desired signal band 3. The lasing mode 11is arranged outside the desired signal band 3. The gain profiles of thecontrol SOA 2 at different bias currents are shown by the lines 30. Thegain profiles 30 of the control SOA 2 overlap the gain profile 29 of thesignal SOA 1 and are significant at the wavelength of the lasing mode11. However, the gain profiles 30 of the control SOA 2 decrease so thatthe gain of the control SOA 2 is insignificant in the desired signalband 3. Typically in the desired signal band 3, the gain of the controlSOA 2 is less than 1%, preferably less 0.1% of the gain of the signalSOA 1, so that the amplification of the signal by the control SOA 2 isnot significant.

The optical amplifier of FIG. 14 operates in exactly the same manner asthe optical amplifiers of the first type described above. In particular,as the gain of the control SOA 2 is significant at the wavelength of thelasing mode, changing the bias current to the control SOA 2 controls theclamped gain of the signal SOA 1 in exactly the same manner. The maindifference is that the control SOA 2 is prevented from amplifyingsignals by having an insignificant gain in the predetermined signal band3, instead of by being arranged outside the signal path 4. This allowsthe control SOA 2 to be arranged in the signal path 4 which has theadvantage of being a simpler arrangement. For example, optical couplersare not essential.

To form an embodiment of a second type, the components of the opticalamplifier of FIG. 14, including both the signal SOA 1 and the controlSOA 2, and optionally also the reflectors 28, may be integrated togetherin a single semiconductor chip 29, as illustrated in FIG. 16.

The optical amplifier of FIG. 16 otherwise has the constructionillustrated schematically in FIG. 14. The above description of both theproperties and the use of the optical amplifier of FIG. 14 appliesequally to the optical amplifier of FIG. 16, although for brevity thatdescription will not be repeated here.

The advantage of the optical amplifier of FIG. 16 is that it allows theentire variable-gain gain-clamped optical amplifier to be integrated ina single semiconductor chip.

The semiconductor chip 29 of FIG. 16 may have the same constructionalong its length as illustrated in FIG. 1 and described above. Lightpasses along the waveguide formed by the active layer 102 passing alongthe entire length of the semiconductor chip 29. Different portions ofthe active layer 102 along the length of the semiconductor chip 29constitute the signal SOA 1 and the control SOA 2.

To allow independent control of the control SOA 1 and the signal SOA 2,the upper contact is separated into two electrically isolated contacts31 and 32, the lower contact 30 being continuous along the length of thesemiconductor chip 29. In use, separate drive currents are independentlysupplied to the upper contacts 31 and 32 causing respective drivecurrents to flow to the lower contact 30 through respective portions ofthe active layer 102 underneath the respective upper contacts 31 and 32,which respective portions of the layer 102 are thus independentlycontrollable.

The reflectors 28 may be integrated in to the semiconductor chip 29beyond the signal SOA 1 and the control SOA 2. Such the reflectors 28may be of conventional construction. Alternatively, the reflectors 28may be formed by separate components outside the semiconductor chip 29.

To provide appropriate properties to the portions of the active layer102 forming the signal SOA 1 and the control SOA 2, it is possible touse conventional manufacturing techniques which selectively modifydifferent regions of the semiconductor chip. Such techniques allow thesignal SOA 1 and/or the control SOA 2 to have a quantum well structure.The energy of the confined states of a semiconductor quantum well areaffected by the width of the quantum well, as well as the band gap ofthe well material and the barrier material.

In particular, the following types of manufacturing method which areknown in themselves for providing a semiconductor chip with differentfunctionality in different regions may be applied to manufacture theoptical amplifier: Shadow-masked growth: This technique is described forexample in “Multiwavelength InGaAs/InGaAsP Strained-Layer MQW-LaserArray Using Shadow-Masked Growth”, IEEE Photonics Technology Letters, 4,524-526 (1992). When layers are grown on a contoured wafer certain areasare partially hidden from the material sources in the growth chamber andhence the thickness of each of the layers varies across the wafer.Therefore the bandgap is varied according to the patterning of the masklayer.

Diffusion enhanced growth: This technique is described for example in“Selective MOVPE growth of InGaAsP and InGaAs using TBA and TBP”, Y.Sakata et al, Seventh International Conference on Indium Phosphide andRelated Materials Proceedings, Pages 839-942 (1995). It is similar toshadow mask regrowth except that the growth is enhanced when atoms areweakly attracted to the mask layer, and can diffuse around on thissurface to the openings in the mask layer. Growth in each window area isthen enhanced due the edges so narrow openings are more stronglyenhanced making the layers that form the quantum wells thicker in theseregions. Quantum-Well Intermixing: This technique is described forexample in “Area Selectivity of InGaAsP-InP Multiquantum-WellIntermixing by Impurity-Free Vacancy Diffusion”, Sang Kee Si et al. IEEEJournal of Selected Topics in Quantum Electronics, 4,619-623 (1998).This is a technique where a material is grown onto the semiconductorwafer that promotes disordering of the material. The layers that formthe quantum wells and the barriers are disordered and the energy of theground state of the quantum well is typically increased.

In the embodiments described above, the laser cavity extendslongitudinally along the signal path of the signal SOA 1. As a result,the laser cavity is very long as compared to the wavelength of thesignal band. Consequently the phase condition for the lasing mode, thatthe net round-trip phase is zero is met at closely spaced intervals, andso the phase condition is not an important design parameter.

There will now be described a third type of embodiment in which thelaser cavity extends transversely across the signal path of asemiconductor optical amplifier, in particular vertically, that isperpendicular to the layered structure of the signal SOA. In this typeof embodiment, the entire optical amplifier is integrated into a singlesemiconductor chip. As a result, the size of the laser cavity becomesshort as compared to the length of the laser cavity. Consequently, thephase condition that the net round-trip phase is zero necessary toestablish the laser mode becomes a significant design parameter.

Such short-cavity devices have particular advantages, because thenatural relaxation oscillation frequency of these lasers is high,typically exceeding the data rate in optical networks, in contrast tothe embodiments described above in which the cavity lengths make thisalmost impossible to achieve. The gain clamping of such short-cavitydevices reacts very quickly, in fact faster than the bit rate, so thatthe gain is clamped dynamically even when the signal power is of thesame order of magnitude as, or perhaps even larger than, the power inthe lasing mode used to clamp the amplification. Thus these devices aremore efficient and can operate closer to their saturation power.

FIG. 17 illustrates a portion of an optical amplifier 50 which isintegrated in a single semiconductor chip with a layered structure andhas the same cross-section along its entire length L perpendicular tothe plane in which the section is taken. The optical amplifier 50 may bemade from material such as InGaAsP, or other suitable semiconductormaterials, in a conventional manner.

The layered structure of the optical amplifier 50 is built up from asubstrate 51 which also constitutes a contact of n-type material.

The optical amplifier 50 has a first layer 52 of active material whichconstitutes a signal active layer 52. The signal active layer 52 may beformed from a bulk material or may have a quantum well structure.

Below and above the signal active layer 52 are respective claddinglayers 53 and 54 to provide confinement of light in the verticaldirection and to form a separate-confinement heterostructure (SCH).

The upper cladding layer 54 is formed with a ridge 55 to provide lateralconfinement of light horizontally across the width W of the opticalamplifier 50, that is parallel to the layered structure andperpendicular to the length L of the optical amplifier 50. The ridge 55in combination with the cladding layers 53 and 54 provides a waveguidefor a laterally guided mode in the approximate position shown by thedotted line 56. In use, the laterally guided mode 56 is the signal whichis amplified by the material of the signal active layer 52 as it passesalong the length L of the optical amplifier 50.

To provide a vertical laser cavity, the optical amplifier additionallyincludes a pair of distributed Bragg reflectors 57 and 58 on either sideof the signal active layer 52. In particular, a lower distributed Braggreflector 57 is provided between the substrate 51 and the lower claddinglayer 53. The upper distributed Bragg reflector 58 is formed on theridge 55 of the upper cladding layer 54 with other layers interposedtherebetween which will be further described below.

The distributed Bragg reflectors 57 and 58 are formed by alternatelayers to provide a periodic structure repeating vertically. The layersof the distributed Bragg reflectors 57 ad 58 extend along the length ofthe optical amplifier 50 to reflect light vertically, thereby forming avertical laser cavity therebetween.

Accordingly, as far as the vertical laser cavity is concerned, theoptical amplifier 50 has a similar structure to a known Vertical CavitySurface Emitting Laser (VCSEL) type of laser, for example as disclosedin “Tunable VCSEL”, C. J. Chang-Hasnain, IEEE J. Select. Topics QuantumElectron, 6,978-987 (2000). Accordingly, the structure of such knownVCSELs may be applied to the vertical laser cavity of optical amplifiersin accordance with the present invention.

Interposed between the upper cladding layer 54 and the upper distributedBragg reflector 58 is a second layer 59 of active material whichconstitutes a control active layer 59. The control active layer 59 isoutside the waveguide 56 formed along the signal active layer 52. As analternative, the control signal layer 59 could be arranged below,instead of above, the signal active layer 52.

To provide independent control of the control active layer 59 and thesignal active layer 52, a first contact layer 60, of p-type material, isdisposed between the two active layers 52 and 59, in particular betweenthe upper cladding layer 54 and the control active layer 59. Also, asecond contact layer 61, of n-type material, is arranged above thecontrol active-layer 59. Therefore, the bias current through the controlactive layer 59 may be controlled by applying a voltage between thefirst contact layer 60 and the second contact layer 61, whereas the biascurrent across the signal-active layer 52 may be controlled by applyinga voltage between the first contact 60 and the substrate 51 which alsoconstitutes a contact.

In use, the lasing action of the laser cavity clamps the total gain ofthe signal active layer 52 and the control active layer 59. Therefore,the signal active layer 52 constitutes the signal active region and thecontrol active layer 59 constitutes the control active region. Theoptical amplifier 50 of FIG. 17 operates in the same manner as theoptical amplifiers described above with the signal active layer 52corresponding to the signal SOA 1 and the control active layer 59corresponding to the control SOA 2.

In particular, the lasing action clamps the total gain of the signalactive layer 52 and the control active layer 59 at the lasing wavelengthat that gain where the lasing threshold is reached, in other words wherethe total gain equals the total losses of the laser cavity. Thus thegain of the signal active layer 52 in the signal band is also clamped.In use, the gain of the control active layer 59 is controlled by varyingthe bias current supplied thereto. The lasing action of the laser cavitycauses the clamped gain of the signal active layer 52 at the lasingwavelength to vary oppositely to the gain of the control active layer59, so that the total gain of the signal active layer 52 and the controlactive layer 59 remains constant. The clamped gain of the signal activelayer 52 imposed on amplification of signals in the signal band iscorrespondingly varied. In other words, control of the control activelayer 59 allows the clamped gain of the signal active layer 52 in thesignal band to be varied without variation of the bias current suppliedacross the signal active layer 52.

This maximizes the saturation output power of the signal active layer52, as compared to a corresponding changing in gain achieved by varyingthe bias current supplied to the signal active layer 52. Changing thebias current supplied to the control of the layer 59 would affect thesaturation output power of the control active layer 59, if light werepassed therealong in the manner of an SOA, but this does not limit thesaturation output power of the signal active layer 52.

In normal use, the bias current supplied to the signal active layer 52is fixed, usually at its maximum value in order to maximize saturationoutput power.

In fact, the gain of the signal active layer 52 may be varied into lossby controlling the bias current supplied to the control active layer 59so as to raise the gain of the control active layer 59 above the laserthreshold of the laser cavity. This is advantageous because there aremany circumstances in telecommunications networks where it is desirableto reduce the level of a data signal, for example to meet the operatingconditions of a particular device. However, such a linear reduction inpower is not possible with a simple SOA.

The graph of FIG. 5 applied equally to the optical amplifier 50 of FIG.17, replacing references to the signal SOA 1 by references to the signalactive layer 52 and replacing references to the control SOA 2 byreferences to the control active layer 59.

The optical amplifier 50 of FIG. 17 may be manufactured by conventionalgrowth techniques, for example epitaxy.

FIG. 18 illustrates a further optical amplifier 65 which has the samestructure as optical amplifier 50 of FIG. 17 except for the additionalprovision of an oxide confinement layer 63. The oxide confinement layer63 is provided as a layer within the ridge 55 of the upper claddinglayer 54, although it could alternatively be provided in one of thedistributed Bragg reflectors 58 close to the active layer 52. The oxideconfinement layer 63 has a central aperture 64 which provides forlateral confinement of the guided mode 56 and also provides for opticalconfinement of the vertical laser mode. This ensures that the lasercavity has a single mode. The oxide confinement layer 63 also providescurrent confinement to ensure that the optical amplifier 50 operatesefficiently. The oxide confinement layer 63 may be formed with thecentral aperture 64 by providing an unoxidized layer extending acrossthe entire width of the ridge 55 and subsequently exposing the edges ofthat layer to oxidize them and create the oxide confinement layer 63with the aperture 64 remaining unoxidized.

The optical amplifier 65 of FIG. 18 is used in the same manner as theoptical amplifier 50 of FIG. 17.

1. An optical amplifier comprising: a signal semiconductor opticalamplifier having a waveguide, forming at least part of a signal pathbetween an input and an output, extending along a signal active regionfor amplification of a signal; a control active region of semiconductormaterial having a gain which is controllable independently from the gainof the signal active region; and a laser cavity containing both thesignal active region and the control active region and being capable ofclamping the gain of the signal active region, wherein the controlactive region is arranged not to amplify a signal in the signal pathwithin a predetermined signal band.
 2. An optical amplifier according toclaim 1, wherein the control active region is the active region of acontrol semiconductor optical amplifier formed separately from thesignal semiconductor optical amplifier.
 3. (Canceled)
 4. An opticalamplifier according to claim 2, wherein the control semiconductoroptical amplifier is outside the signal path so that it does not amplifya signal in the signal path.
 5. An optical amplifier according to claim4, wherein the laser cavity extends longitudinally along the signalpath.
 6. An optical amplifier according to claim 5, wherein the lasercavity is a ring cavity.
 7. An optical amplifier according to claim 6,wherein the ring cavity includes an isolator which controls thepropagation direction of the lasing mode in the laser cavity withrespect to the signal path.
 8. An optical amplifier according to claim6, further comprising a pair of optical couplers in the signal path onthe input side and output side, respectively, of the signalsemiconductor optical amplifier coupling the ring laser cavity into thesignal path.
 9. An optical amplifier according to claim 8, wherein thepair of optical couplers couple respective ends of an optical path whichcontains the control semiconductor optical amplifier and which, togetherwith the portion of the signal path between the pair of opticalcouplers, forms the ring laser cavity.
 10. An optical amplifieraccording to claim 5, wherein the laser cavity is a linear cavity. 11.An optical amplifier according to claim 10, further comprising anoptical coupler in the signal path coupling the linear laser cavity intothe signal path.
 12. An optical amplifier according to claim 11, whereinthe optical coupler couples an end of an optical path which contains thecontrol semiconductor optical amplifier and which is terminated by areflector to form one end of the linear laser cavity.
 13. An opticalamplifier according to claim 12, wherein the other end of the linearlaser cavity is terminated by a reflector arranged in the signal path.14. An optical amplifier according to claim 12, wherein the other end ofthe linear laser cavity is formed by a second optical coupler coupled inthe signal path, on the opposite side of the control semiconductoroptical amplifier from the first mentioned optical coupler, to a secondoptical path terminated by a reflector.
 15. An optical amplifieraccording to claim 8, wherein each optical coupler is a wavelengthdivision multiplexing coupler or other wavelength-selective coupler. 16.An optical amplifier according to any one of claim 8, wherein eachoptical coupler is a wavelength-insensitive coupler.
 17. An opticalamplifier according to claim 1, wherein the signal active region and thecontrol active region are different regions of the same semiconductorchip, said waveguide extending along both the signal active region andthe control active region.
 18. An optical amplifier according to claim1, wherein the control active region is integrated outside the waveguidein the same semiconductor chip as the signal semiconductor opticalamplifier.
 19. An optical amplifier according to claim 18, wherein thelaser cavity extends transversely to the waveguide.
 20. An opticalamplifier according to claim 19, wherein the laser cavity extendsperpendicular to the layered structure of the semiconductor chip.
 21. Asemiconductor optical amplifier according to claim 19, wherein thesignal and control active regions are formed by respective layers ofactive material and the laser cavity extends perpendicular to thelayers.
 22. A semiconductor optical amplifier according to claim 21,further comprising contact layers on both sides of the signal andcontrol active layers for independently controlling the bias currentacross each active layer.
 23. An optical amplifier according to claim 1,wherein the laser cavity has a lasing mode at a wavelength outside thepredetermined signal band.
 24. An optical amplifier according to claim23, wherein the laser cavity includes a wavelength-dependent element tocontrol the wavelength of the lasing mode.
 25. An optical amplifieraccording to claim 24, wherein the wavelength-dependent element is afilter in the laser cavity outside the signal path.
 26. An opticalamplifier according to claim 24, wherein the wavelength-dependentelement is a wavelength-selective coupler.
 27. An optical amplifieraccording to claim 1, wherein the control active region has aninsignificant gain in the predetermined signal band so that it does notamplify a signal in the predetermined signal band.
 28. An opticalamplifier according to claim 27, wherein the control active region is inthe signal path.
 29. An optical amplifier according to claim 28, whereinthe laser cavity is formed between reflectors arranged in the signalpath.
 30. An optical amplifier comprising: at least a signal and acontrol semiconductor optical amplifier in an optical circuit, whereinthe optical circuit comprises: a signal path passing through the signalsemiconductor optical amplifier; and a laser cavity containing thesignal and control semiconductor optical amplifiers with the controlsemiconductor optical amplifier outside the signal path.
 31. An opticalamplifier comprising: at least a signal and a control semiconductoroptical amplifier in a signal path; and a laser cavity containing thesignal and control semiconductor optical amplifiers, the controlsemiconductor optical amplifier having an insignificant gain in apredetermined signal band.
 32. A semiconductor optical amplifiercomprising: a signal active region having a waveguide extendingtherealong to act as a signal path; a control active region outside thewaveguide and controllable independently from the signal active region;a laser cavity containing both the signal active region and the controlactive region.
 33. A method of controlling the gain on the signal pathof an optical amplifier according to claim 1, the method comprisingcontrolling the gain of the control active region.
 34. A methodaccording to claim 33, wherein the gain of the control active region iscontrolled by controlling the bias current to the control active region.